The mammalian Nit1 protein is homologous to bacterial and plant nitrilases. Previous studies have shown that Nit1 plays a role in wound and herbicide induced apoptosis. In flies and worms Nit1 is fused to the 5'end of the Fhit tumor suppressor. Therefore, it has been suggested that Nit1 may interact functionally with the Fhit pathway. The Fhit protein is a tumor suppressor. Somatic loss of Fhit is associated with a wide variety of cancers. Deletion of Fhit results in predisposition to spontaneous lymphoid malignancy in mice. There was also evidence showing that Nit1 deficiency lead to increased incidence of induced tumors in mice. The molecular mechanism involved in the tumor suppressing function of Fhit and Nit1 is not clear. Previous studies have shown that ectopic expression of Fhit or Nit1 can induce apoptosis in tumor cells, which requires caspase activation but is independent of Bcl-2 and Bcl-xL. Furthermore, Fhit- induced apoptosis can be inhibited by dominant negative mutant FADD. We and others have previously demonstrated that FADD is a mediator of the extrinsic apoptotic signaling pathways initiated by death receptors. Apoptosis induced by the Fas death receptor plays an important role in homeostasis in the immune system. To help understand the physiological function of Nit1 in T cells, we have performed preliminary analyses of Nit1 knockout mice. These mutant mice contained normal numbers of thymic and peripheral T cell sub populations. Our data showed that Nit1-/- T cells had a mild defect in Fas and Cainduced apoptosis. Although Nit1 may play a role in DNA-damage induced apoptosis in kidney cells, our preliminary data indicated that that apoptosis induced by a variety of stimuli including DNA damages due to 3- irradiation or chemical treatments was not affected in Nit1-deficient T cells. Unexpectedly, we found that Nit1 deficiency resulted in hyperproliferative responses in T cells induced through the antigen receptor stimulation. In this application, we propose an array of biochemical, genetic and immunological analyses to help understand the novel function of Nit1 in negatively regulating T cell proliferation. Specifically, our aims are (1) to analyze the function of Nit1 in T cell apoptosis;(2) to understand the non-apoptotic function of Nit1 in T cell proliferation;(3) to determine the functional domains of Nit1 by reverse genetics. PUBLIC HEALTH RELEVANCE: In this application, we describe a scientific plan to study the Nit1 protein which plays an essential role in immune system functions, and cancer. The results will facilitate the development of effective preventive and therapeutic approaches.